#ifndef STAN_MATH_PRIM_SCAL_FUN_INV_LOGIT_HPP
#define STAN_MATH_PRIM_SCAL_FUN_INV_LOGIT_HPP

#include <cmath>

namespace stan {
  namespace math {

    /**
     * Returns the inverse logit function applied to the argument.
     *
     * The inverse logit function is defined by
     *
     * \f$\mbox{logit}^{-1}(x) = \frac{1}{1 + \exp(-x)}\f$.
     *
     * This function can be used to implement the inverse link function
     * for logistic regression.
     *
     * The inverse to this function is <code>logit</code>.
     *
     *
     \f[
     \mbox{inv\_logit}(y) =
     \begin{cases}
     \mbox{logit}^{-1}(y) & \mbox{if } -\infty\leq y \leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } y = \textrm{NaN}
     \end{cases}
     \f]

     \f[
     \frac{\partial\, \mbox{inv\_logit}(y)}{\partial y} =
     \begin{cases}
     \frac{\partial\, \mbox{logit}^{-1}(y)}{\partial y} & \mbox{if } -\infty\leq y\leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } y = \textrm{NaN}
     \end{cases}
     \f]

     \f[
     \mbox{logit}^{-1}(y) = \frac{1}{1 + \exp(-y)}
     \f]

     \f[
     \frac{\partial \, \mbox{logit}^{-1}(y)}{\partial y} = \frac{\exp(y)}{(\exp(y)+1)^2}
     \f]
     *
     * @param a Argument.
     * @return Inverse logit of argument.
     */
    inline double inv_logit(const double a) {
      using std::exp;
      return 1.0 / (1.0 + exp(-a));
    }

  }
}
#endif
